In the production of machine parts, castings of iron or steel are frequently used due to their Near-Net-Shape (NNS) capability, reducing the necessary amount of material to be removed by machining, thus promoting Lean Production and reducing both energy consumption and environmental impact. Iron castings containing graphite inclusions with spherical, vermicular or lamellar shapes further improve both the castability and the machinability of the machine parts in comparison to steel castings, and iron castings are therefore preferred if their mechanical properties are sufficient for a particular application. Two main disadvantages with castings, which can result in undesired non-uniformity or scatter in their mechanical properties, are the inherent presence of porosity, at least on a microscopic level, and the segregation of elements in comparison to the equalization in rolled or forged steel, which is particularly detrimental during heat treatments of the castings.
Ductile iron (also called nodular cast iron) is a cast iron that contains carbon in the form of graphite spheroids/nodules. Due to their shape, these small spheroids/nodules of graphite cause less severe stress concentrations in the continuous matrix (actually having a steel composition) compared to the finely dispersed graphite flakes in grey iron, thereby improving strength and in particular ductility as compared with other types of iron.
Austempered ductile iron (ADI) (which is sometimes erroneously referred to as “bainitic ductile iron” represents a special family of ductile iron alloys which possess improved strength and ductility properties as a result of a heat treatment called “austempering”. The heat treatment produces a duplex matrix microstructure named “ausferrite” consisting of acicular ferrite precipitated in carbon-stabilized austenite.
ADI castings are, compared to conventional ductile iron, at least twice as strong at the same ductility level, or show at least twice the ductility at the same strength level. Compared to steel castings of the same strength, the cost of casting and heat treatment for ADI is much lower, and simultaneously the machinability is improved, especially if conducted before heat treatment. High-strength ADI cast alloys are therefore increasingly being used as a cost-efficient alternative to welded structures or steel castings, especially since components made from steel are heavier and more expensive to manufacture and to finish than components made from ADI.
Ausferritic steels can be obtained by similar heat treatments as for ausferritic irons, on condition that the steels contain sufficient silicon to prevent the precipitation of carbides. The main difference with respect to irons is that in steel the carbon content is approximately constant in the iron-based matrix, while in irons it can be varied by the selection of the austenitization temperature during heat treatment. One of the rolled steels being suitable for austempering is the spring steel EN 1.5026 with typical composition 0.55 weight-% carbon, 1.8 weight-% silicon and 0.8 weight-% manganese.
The segregation of alloying elements that are added for hardenability is more pronounced in castings than in rolled or forged steel, where the plastic deformation equalizes the compositional variations. It has been shown that when using elements that improve hardenability, such as manganese or molybdenum, the “positive” segregation, (i.e. the segregation that occurs at a late stage during solidification) of larger amounts of these elements in intercellular volumes of cast iron or cast steel, is detrimental for the completion of acicular ferrite precipitation during the formation of ausferrite. The consequence is that austenite in the remaining intercellular volumes, being unaffected by the beneficial enrichment of carbon associated with the precipitation of acicular ferrite, will then not be stabilized against transformation to martensite during the final cooling. Increasing the hardenability by other means than by these additives would therefore be advantageous.
In a typical austempering heat treatment cycle, work pieces comprising iron or steel are firstly heated and then held at an austenitizing temperature until they become fully austenitic. In the case of cast irons, where the graphite inclusions provide a degree of freedom regarding carbon content in the matrix, the austenite must also be given enough time to be saturated with carbon diffusing from the graphite and, if the iron contains pearlite, also with carbon from the dissolution of its cementite. After the work pieces are fully austenitized, they are quenched (usually in a salt bath) at a quenching rate that is high enough to avoid the formation of pearlite during the quenching down to an intermediate temperature above the temperature Ms, at which the austenite having this level of carbon would otherwise start to transform into martensite. This intermediate temperature range is better known as the bainitic range for common low-silicon steels, and in a similar way the ausferritic microstructure becomes either coarser for higher transformation temperatures, but here with a larger amount of austenite (promoting higher ductility), or finer for lower temperatures with a larger amount of ferrite (enabling higher strength). The work pieces are then held for isothermal transformation to ausferrite at this temperature called the austempering temperature, followed by cooling to room temperature.
The superior mechanical properties of ausferritic materials emanate from an ausferritic microstructure of very fine needles of acicular ferrite in a matrix of austenite, thermodynamically stabilized by the concurrent enrichment of carbon to a high carbon content. The much higher silicon content in austempered ductile irons, compared to common steels, stabilizes carbon in graphite instead of cementite (Fe3C), thus preventing the precipitation of bainitic carbides as long as the austempering is not too prolonged.
U.S. Pat. No. 5,522,949 discloses a method for improving the mechanical properties, such as tensile strength, yield strength and fracture elongation of a ductile iron, by subjecting the ductile iron to Hot Isostatic Pressing before it is subjected to a conventional austempering treatment.
Hot Isostatic Pressing (HIP) is a process that is used to reduce the porosity of metals and to influence the density of ceramic materials. The HIP process subjects a work piece to both elevated temperature and isostatic gas pressure (whereby pressure is applied to the material from all directions) in a high pressure containment vessel. An inert gas such as argon is usually used to prevent chemical reactions, and the pressurizing gas is usually raised to a pressure level between 100-300 MPa by a combination of pumping and electrical heating of the gas surrounding the work pieces. When materials are treated with HIP, the simultaneous application of heat and pressure eliminates internal (closed) voids and microporosity through a combination of plastic deformation, creep, and diffusion bonding.
While resulting in the production of austempered material having improved mechanical properties, the use of Hot Isostatic Pressing before a conventional austempering treatment substantially increases manufacturing time and costs.